1. Field of the Invention
The present invention relates to a gauge for calibrating a three-dimensional coordinate measuring machine (hereinafter referred to as CMM), that is used in examining the accuracy of the CMM and to a method for calibrating the CMM using the gauge.
2. Description of the Prior Art
A CMM is a machine for measuring the dimensions and shape of a substance under measurement using coordinate points interspersed in a three-dimensional space with the aid of a computer. To be specific, the substance under measurement disposed on a surface plate and a probe attached to the tip of a Z-axis in the CMM are relatively moved in three-dimensional directions, moments of contact of the probe with the substance are ascertained, coordinate values in the directions of the moving axes are read using the moments as electrical triggers, and the dimensions and shape of the substance are measured using the computer.
Generally, CMMs are required to measure a substance with particularly high accuracy. In order to guarantee high-accuracy measurement, a CMM has to be subjected to accuracy examination before every use, and values obtained by the measurement with the CMM are calibrated using the accuracy examination results as calibration values or adjusting means is used to finely adjust the CMM. This accuracy examination requires use of a gauge as the standard. The gauge is required to enable evaluation of values detected by a CMM""s probe when it is moved three-dimensionally.
A first important target for researchers was how the error of each axis in the CMM should be measured. Therefore, a gauge was first invented for the purpose of measuring such errors in the CMM. It is now widely known that, fundamentally, the errors should be measured by measuring a sphere or spheres. For this reason, research has turned to a second target of determining how the sphere or spheres should be disposed in what mode to form a gauge for measurement and evaluation. Various attempts have been made to dispose the spheres in one same plane and dispose them sterically.
A gauge of this type for calibrating a CMM is disclosed in JP-UM-A HEI-1-64004 and shown in FIG. 8. As shown, this gauge comprises a block body 51 in the form of a flat rectangular parallelepiped, a plurality of substantially cubic index projections 52 for accuracy examination arranged at regular intervals on the upper surface 51a along one end face 51b of the block body 51, and a plurality of substantially hemispherical projections 53 for repeated accuracy examination integrally formed at predetermined positions on the upper surface 51a of the block body 51. The accuracy examination is carried out by setting the block body 51 in place on a table for the CMM, bringing the probe of the CMM into contact successively with the index projections 52, for example, to read the indices of the CMM, and using the indices and the intervals between the projections 53 for repeated accurate examination to effect computation. Various kinds of gauges similar to the gauge mentioned above have been put to practical use for CMM calibration.
Although consideration has been given to how spheres should be disposed in one same plane and how spheres should be disposed three-dimensionally as described above, since no national standard for an intercentral distance of spheres has yet been established, the intercentral distance of spheres cannot be expressed in terms of submicrometer units irrespective of how the spheres are disposed. Only the intercentral distance of the spheres not expressed in sub-micrometer units and the diameters of the spheres obtained by calculation from the intercentral distance can be measured. In the conventional CMM calibrating gauge shown above, moreover, standard values are obtained by measuring the gauge parts as accurately as possible. Thus, the standard values are merely measured values inherently containing errors and therefore lack reliability.
In view of the above, the principal object of the present invention is to provide a CMM calibrating gauge that enables total calibration of errors in graduations of each axis and operational performance of CMMs.
Another object of the present invention is to provide a CMM calibrating gauge that enables a plurality of revisions of the measured values and accurate calibration of CMMs of any size.
Still another object of the present invention is to provide a CMM calibrating gauge easy to manage, store and maintain.
Yet another object of the present invention is to provide a method for specifying the three-dimensional dimensions of a CMM calibrating gauge with high accuracy and for accurately calibrating a CMM including the motion characteristics of the CMM in a specific direction.
To attain the objects described above, the present invention provides a CMM calibrating gauge having a fundamental structure that comprises a block gauge having a pair of opposite end faces whose absolute length values have been certified and at least one sphere fixed to a front surface of the block gauge.
The at least one sphere may be a plurality of spheres. These spheres may have different sizes. One or more spheres may also be fixed to the back surface of the block gauge.
The CMM calibrating gauge can be used alone or in combination with an optional number of like block gauges and/or an optional number of like CMM calibrating gauges connected at their respective end faces.
The present invention further provides a method for calibrating a CMM using the CMM calibrating gauge of the fundamental structure, which comprises the steps of bringing a probe of the CMM into contact with a first end face of the block gauge of the CMM calibrating gauge to specify planarity of the first end face, bringing the probe into contact with a peripheral surface and a pole point of the sphere to specify coordinates of a sphere center relative to the first end face and a diameter of the sphere, bringing the probe of the CMM into contact with a second end face of the block gauge of the CMM calibrating gauge to measure planarity of the second end face, bringing the probe into contact with the peripheral surface and the pole point of the sphere to measure coordinates of the sphere center relative to the second end face and the diameter of the sphere and revise the specified planarity of the first end face and the specified sphere center coordinates and sphere diameter, thereby specifying three-dimensional dimensions of the CMM calibrating gauge to calibrate the CMM.
When a combination CMM calibrating gauge comprising the aforementioned CMM calibrating gauge and an optional number of like block gauges and/or an optional number of like CMM calibrating gauges is used, the three-dimensional dimensions of the combination CMM calibrating gauge and the CMM including its motion characteristic in a specific direction can be calibrated with higher accuracy.
The above and other objects, features and advantages of the present invention will become apparent from the description given hereinbelow with reference to the accompanying drawings.